The present invention relates to a method for the recovery of the chromic acid component from the chromium plating waste water obtained by washing chromium plated products and also relates to a method of treatment for converting the chromium plating waste water into an innocuous solution.
In general, in methods for the treatment of chromium plating waste water with the aid of ion exchange resins, mainly direct ion exchange resin processes have been widely employed to date. In those processes, suspended substances are first removed by means of a filter, and the filtrate is allowed to pass through a cation exchange resin tower for adsorption and removal of the cationic components and is then allowed to pass through an anion exchange resin tower for adsorption on the dichromic acid ions; when the anion exchange resin is saturated, the dichromic acid ions are desorbed with sodium hydroxide in the form of sodium chromate solution, which is then treated in the cation exchange resin tower so as to remove the sodium ions and thereby recover dichromic acid liquor. In such processes, however, thorough adsorptive removal of cationic components in the pretreatment with the first cation exchange resins does not occur, and thus, some metallic components are deposited on the resins during the course of the subsequent treatment with anion exchange resins, which impairs the ion exchange efficiency of the resins due to clogging and also impairs the flow rate, to mention only a few of the drawbacks.
The present inventors have thus devised a so-called neutralization type ion exchange resin process to eliminate the drawbacks described above. In this process, an alkaline component is first added to the chromium plating waste water until it is almost neutral so as to convert all the metallic components contained therein into hydroxides, which are then removed by filtration; the filtrate so obtained is then acidified by the addition of dilute sulfuric acid and is allowed to pass through an anion exchange resin tower to adsorb dichromic acid ions; when the resin is saturated, the dichromic acid ions are desorbed in the form of sodium chromate by the use of a sodium hydroxide solution; further, the solution is treated in a cation exchange resin tower for the removal of sodium ions in order to recover dichromic acid liquor. Detailed studies by the present inventors, however, have proved that, in such processes, in the step of adsorbing dichromic acid ions in the anion exchange, resin tower, from filtrate which has been acidified by the addition of dilute sulfuric acid (the purpose of acidifying the filtrate is to improve the absorbing power of the anion exchange resins for chromium acid), the sulfuric acid ions are also adsorbed on the anion exchange resins and the dichromic acid liquor finally obtained contains a large amount of sulfuric acid, thus making the recycling of the liquor as such as a plating solution unfeasible. In order to make the recycling of the liquor as a plating solution possible, the sulfuric acid component has to be removed by the use of calcium hydroxide or barium hydroxide, but full removal of the sulfuric acid component is not effected by calcium hydroxide and the extremely high price of barium hydroxide means that its use for the treatment of the waste water is not feasible from an economic standpoint. It is thus desirable to establish a method for the recycle treatment of chromium plating waste water, wherein the step of removal of the sulfuric acid component has been eliminated.